Mode Locking of Quantum Cascade Lasers
Final rept. 14 Jun 2004-13 Jun 2007
HARVARD UNIV CAMBRIDGE MA SCHOOL OF ENGINEERING AND APPLIED SCIENCES
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A theoretical and experimental study of multimode operation regimes in quantum cascade lasers QCLs is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes in QCLs One is spatial hole burning SHB, and the other one is related to the Risken-Nummedal-Graham-Haken RNGH instability predicted in the sixties. A model that can account for coherent phenomena, a saturable absorber and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with narrow active region andor with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have weaker saturable absorber, do not exhibit a Rabi splitting, and their multimode regime is governed by SHB. This experimental phenomenology is well explained by our theoretical model.
- Lasers and Masers